It is not unusual that when it rains, it will dissolve surface materials or carny it off as suspended materials into steams and such. Each time it rains, runoff carries an earthy tea steeped from leaf litter, crop residue, soil, and other organic materials into the storm drains and streams that feed Chesapeake Bay or many other bodies of water. Apparently some sources of organics are worse than others. A new study led by researchers at William & Mary's Virginia Institute of Marine Science reveals that land use in the watersheds from which this dissolved organic matter originates has important implications for Bay water quality, with the organic carbon in runoff from urbanized or heavily farmed landscapes more likely to persist as it is carried downstream, thus contributing energy to fuel low-oxygen dead zones in coastal waters.

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The new study appears in this month's issue of the Journal of Geophysical Research, and was highlighted by the journal's publisher, the American Geophysical Union, as an AGU Research Spotlight in their print and online channels.

Low-oxygen dead zones are a growing problem in Chesapeake Bay and coastal ecosystems worldwide. While most management practices focus on reducing inputs of nitrogen and other nutrients known to fuel dead zones, Canuel says "organic matter from the watershed may also contribute. One goal of our study was to examine the quality of organic matter derived from streams and its potential to contribute to dead-zone formation."

In the 1970s, the Chesapeake Bay was discovered to contain one of the planet's first identified marine dead zones, where hypoxic waters were so depleted of oxygen that they were unable to support life, resulting in massive fish kills. This results in part from large algal blooms, which are nourished by the runoff of residential, farm and industrial waste throughout the watershed.

As streams and rivers carry dissolved organic matter downstream, bacteria or sunlight can modify it into compounds and forms that are more difficult for organisms to use. While the team's research showed no significant difference in bacterial degradation of organic matter from cleared or forested watersheds, Canuel says it did show that "organic carbon in runoff from watersheds affected by human activity is less susceptible to solar degradation than that from forested watersheds."

Urban organics thus remain at higher levels longer, says Canuel, "delivering more organic material to the river mouth and increasing the likelihood that low-oxygen conditions will develop in downstream locations such as estuaries and the coastal ocean."

The authors aren't yet sure why the organic carbon from the more developed watersheds is less vulnerable to breakdown by sunlight in rivers and streams, but suggest that it might be because it has already been exposed to appreciable sunlight in the less shady urban and agricultural environment.

Says Canuel, "Urban organics may persist downstream because their more photoreactive compounds have already been degraded due to greater light exposure in urban areas, farm fields, and pastures, leaving only the more photo-resistant, refractory compounds to wash into the coastal zone."

"Our results show that future studies should assess not only the quantity of dissolved organic carbon entering our rivers and streams, but also its source," says Canuel. "Understanding how organic matter from developed and undeveloped watersheds behaves in the aquatic environment will contribute to the development of more effective watershed management practices and hopefully more successful efforts to reduce the number, extent, and duration of low-oxygen dead zones."